Due to material technology, the highest allowable temperature of the pn-junction of a power semiconductor component is fixed. This temperature cannot be exceeded without risking the reliable operation of the component. The junction temperature is affected by ambient temperature and also by the magnitude of losses in the semiconductor. The losses in the semiconductor component can be divided into conduction losses and switching losses, both of which are dependent on the current flowing through the component. The magnitude of switching losses is greatly dependent on the switching frequency and on the voltage over the component when in blocking state. The losses in the semiconductor component can be reduced by reducing the current flowing through it. For this reason it is possible to limit the junction temperature of the semiconductor component to a desired level by limiting the current flowing through it to suitable level.
One straightforward method for protecting power semiconductors is to define a current limit that is either fixed or possibly dependent on the switching frequency that prevents the excessive heating of the power semiconductors. A drawback of this method is that it does not take into account the temperature of the semiconductor module and thus also limits the current in the situation where the temperature of the module is low. Further, the above method does not take into account the output frequency, which leads to deficient protection at low output frequencies or alternatively to an unnecessarily low current limit at the higher output frequencies.
An advanced solution is to estimate the junction temperature using thermal resistances and thermal capacitances in a thermal model of a power semiconductor component. Estimated losses in the component are the inputs to the thermal model. Document R. Schnell, U. Schlapbach (ABB Switzerland Ltd., Semiconductors), “Realistic benchmarking of IGBT-modules with help of a fast and easy to use simulation-tool,” proc. PCIM'04, 2004, shows that so called Zach equations can be used to estimate the losses needed for the thermal model. Zach equations can be used in the estimation of conduction losses of IGBTs and diodes when the converter is operating with a known modulation index and known output current. Switching losses can be estimated from data relating to switching loss energies given by the component manufacturer.
The losses of an IGBT bridge can be estimated with even more precise methods than the above-mentioned Zach equations. The switching losses of an IGBT bridge of a frequency converter can be estimated for example by calculating state changes, i.e. switchings of separate IGB transistors, and by determining switching loss powers relating to the switchings from the data provided by the component manufacturer. The conduction losses can be estimated by determining continuously which one of the semiconductor components, i.e. diode or IGBT, is in the conducting state, and by using a suitable conduction state loss model for the conducting component.
The above method includes the use of a thermal model which tries to depict the junction temperature of the semiconductor. When the junction temperature raises high enough, the output current and/or switching frequency of the converter must be limited. The above method includes the following three clearly distinct entities:                1. Loss model for the determination of losses        2. Thermal model for the calculation of the junction temperature        3. Logic for lowering the output current and/or switching frequency depending on the calculated junction temperature        
All the above entities are implemented in the control circuitry of the frequency converter and must be processed in real-time during the operation of the frequency converter. This leads to a relatively complex procedure for the protection of power semiconductors.
U.S. Pat. No. 4,980,624 discloses a method for thermal protection of a motor converter and of a motor. In this method the loading of the converter and motor is determined using motor current. Further, in the determination of the loading of the converter, the switching frequency is taken into account and in the determination of the loading of the motor, the output frequency is taken into account. The purpose of the method of U.S. Pat. No. 4,980,624 is to identify an overload situation and to trip the drive.
Document JP2000228882 A discloses a protection method for a frequency converter, which method tries to estimate the highest junction temperature of a power semiconductor. In this estimation the magnitude of the output current, switching frequency, output frequency, measured temperature and parameters depicting the dynamic temperature behavior of the parts of the converter are used in the estimation of junction temperature. In this method the junction temperature is estimated in real-time during the operation of the frequency converter.
One of the disadvantages associated with the above methods is the use of a thermal model that needs to be calculated during the operation of the frequency converter. These models are relatively complex and require high processor capacity.